It has been found that upon curing of a portland cement mortar or concrete composition containing a film-forming polymeric modifier, e.g. a polymer latex, a three-dimensional network of polymeric film is formed throughout such material. This film seals the material and prevents the evaporation of moisture therefrom by buffering such composition from the deleterious effects of low humidity, wind and excessive temperatures. Under extreme drying conditions, such as high wind and/or high temperature combined with low humidity conditions, the modified concrete may develop crazing or random surface cracking approximately one-tenth to one-half of an inch deep. Such cracking is believed to be caused by the following sequence of events: (1) the surface layer of the latex modified cement mortar or concrete loses water rapidly through surface evaporation, (2) coalescence of the polymer modifier (latex) is induced by drying causing the polymer in the surface layer to form a film network before the cement paste, which is the matrix of such network, has had a chance to develop mechanical strength through the hydration reaction, (3) the hydration reaction proceeds to a point wherein the cement paste phase undergoes volume shrinkage exerting tensile stress, and (4) since the polymer modifier has already formed a network in the surface layer and resists shrinkage stress, when the stress exceeds the tensile strength of the polymer network such stress is relieved by cracking of the surface layer.
While numerous liquid curing membranes have been proposed to prevent the excessive loss of moisture from portland cement mortar and concrete compositions, none of these compounds have proven to perform satisfactorily with a portland cement mortar or concrete composition containing a film-forming polymeric modifier. The primary reason for such failure is that all existing curing compounds are designed to be applied after the initial set of the portland cement mortar or concrete surface. This occurs primarily because most of the compositions contain a water-immiscible solvent which is incompatible with free water on the surface. In polymer modified portland cement mortar or concrete, however, the crack begins to form before the initial set.
It is, therefore, a primary object of the present invention to provide a means for the prevention of surface cracking of portland cement mortar or concrete containing a film-forming polymeric modifier.